The following patents and other patent documents are believed to represent the current state of the art:    U.S. Pat. Nos. 5,539,393; 5,553,412; 5,572,653; 5,753,900; 5,880,449; 5,933,813; 5,975,416; 6,031,585; 6,081,558; 6,081,612; 6,085,576; 6,091,884; 6,099,522; 6,104,939; 6,105,004; 4,002,886; 4,409,470; 4,766,295; 5,313,569; 5,382,779; 4,521,677; 4,782,219; 4,139,149; 5,345,071; 5,457,307; 4,387,297; 4,760,248; 5,019,811; 5,361,871; 3,080,440; 3,876,470; 4,041,221; 5,089,107; 5,833,934; 5,846,669; 5,968,325; 6,162,267;    JP 2000152857; DE 19840248; DE 19840251; EP 974918A2; AU 9923674; FR 2778775; FR 2772529; FR 2778775; SE 9103048; RD 344094; GB 2249854; EP 0837439;    WO 95/22798; WO 98/00819; WO 98/32115; WO 99/65288; WO 96/32683; WO 98/32115.
The present applicant/assignee is the proprietor of the following published patents and patent applications:    U.S. Pat. Nos. 6,205,396; 5,880,449; 5,933,813; 6,105,004; 4,113,921.    PCT-IL-97/00339 and PCT-IL-97/00192.    WO 98/00819; WO 96/32683.    Reference is also made to “A Methanol Impermeable Proton Conducting Composite Electrolyte System”, Cong Pu et al, Journal of the Electrochemical Society Vol. 142, No. 7 Jul. 1995;    “PT2000 Portable Terminal”, pp 1-2; http://www.percon.com/html/pt2000.htm Buckle, J (editor) “Sending Out Clear Signals”, European Supermarkets, March/April 1999, pp 26-32; and    Research Disclosure 34494. “Electronic Price Tag”, Anonymous, December 1992, pp 969.